<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>

<body>
    <script>
        function getDayNum(dataStr) {
            var currentTime = +new Date(dataStr);
            var startTime = +new Date("2019-1-1");
            var days = Math.floor((currentTime - startTime) / 1000 / 60 / 60 / 24) + 1;
            return days;
        }
        console.log(getDayNum("2019-1-3"));

    </script>
</body>

</html>